Technical Field
The present application relates to radio frequency identification (RFID) systems and, in particular, to echo cancellation within such systems.
Background Art
In a backscatter-based RFID system, the reader broadcasts a continuous-wave carrier signal. The reader may modulate the continuous-wave carrier signal to send an information signal to any transponders (also called ‘tags’) in range. A nearby transponder sends a reply signal by modulating the impedance of its receive antenna. In very general terms, the backscatter modulation may involve switching the transponder antenna between a short-circuit condition and an open-circuit condition to change the reflective/absorptive characteristic of the load seen by the continuous-wave carrier signal. The reader includes a receive antenna (which may be the same as the transmit antenna in many cases), that receives the reflected continuous-wave carrier signal.
Some backscatter transponders lack an internal power source, like a battery, and rely upon the energy of the received carrier signal to power the transponder circuitry. These are generally termed “passive” RFID systems. Backscatter modulation is common in passive RFID systems, but may also be used in “active” RFID systems in which the transponder has its own power source, e.g., a battery.
FIG. 1 shows a simplified block diagram of an example reader 10 for a backscatter-based RFID system. The reader 10 includes a transmitter section 12, a receiver section 14, a circulator 16, and an antenna 18. The transmitter section 12 in this example includes a signal generator for generating the carrier wave at an RF frequency, an amplifier, and, optionally, a modulator for modulating the carrier wave with an information signal. The carrier wave (modulated or un-modulated) is broadcast using the antenna 18. A reflected wave from a nearby transponder induces a signal in the antenna 18 that is received by the receiver section 14, where it is downconverted using the carrier wave, amplified, filtered, and then demodulated to recover whatever signal the transponder imposed on the reflected carrier wave.
There are a number of sources of leakage of the transmitter section 12 into the receiver section 14 in the example reader 10. Leakage signals caused by transmissions that reach the receiver section 14 may be referred to as “echo” signals, although not all types constitute actual reflection echo signals. Some example sources include (a) leakage from transmitter section 12 to receiver section 14 through the circulator 16, (b) reflections due to RF mismatches in the RF path to the antenna 18 (assuming a common antenna is used for transmitting/receiving), (c) coupling between antennas (in the case of separate transmit and receive antennas), and (d) reflections from objects in view of the antenna 18, including moving objects like vehicles. Any or all of these sources may result in unwanted RF-level signal in the receiver resulting from the transmitter.
It will be noted that the receiver section 14 downconverts the received signal using a carrier-wave frequency shift, and then uses a high-pass filter to eliminate the carrier-wave frequency signal that has been downconverted to baseband, leaving just the transponder information signal. However, large unwanted echo signal can be large enough to degrade the reader 10 performance. For example, the echo signal may be larger than the amplifier or downconverter can tolerate. Furthermore, the phase noise of the unwanted echo signal may be large enough to degrade the modulation sideband of the transponder information signal, causing the demodulator to fail to properly recover the transponder information at the minimum required power.
U.S. Pat. No. 6,192,222 to Greef et al. and U.S. Pat. No. 7,986,931 to Lin et al. both describe echo cancellation circuits for an RFID reader that are intended to remove leakage signal from a backscatter modulated signal. The system described by Greef et al. uses a variable attenuator and a variable phase shifter to create a signal that is then subtracted from the received RF-level signal, so as to minimize the power level of the carrier wave frequency component in the received signal. The system described by Lin et al. involves multiplying the carrier wave signal by a complex gain factor and then subtracting the resulting RF signal from the received RF-level signal, so as to minimize the power level of the carrier wave frequency component in the received signal.
Unfortunately, both the systems of Greef et al. and Lin et al. employ search algorithms and require many iterations to converge and are not practical for use in systems where speed is important.
Similar reference numerals are used in different figures to denote similar components.
Embodiments will now be described with reference to the accompanying drawings. Similar reference numerals are used in different figures to denote similar components